The invention relates to a method for feeding a treating agent onto a moving surface, and an apparatus for feeding a treating agent onto a moving surface. The moving surface can be formed of a web, a roll shell or a belt or an equivalent moving member by means of which a treating agent is transferred to the web. The web to be treated, in turn, can be a paper web, a board web in a paper or board machine or a plastic film. In this application, by the treatment of the web is meant surface-sizing, coating or another equivalent process of treating a web in which a treating agent is added at least to one surface layer of the web.
Surface sizing and coating methods can be divided roughly into contact-based methods and non-contact methods.
The contact-based methods include, for example, blade coating, film transfer coating and air knife coating. In blade coating, a treating agent is applied to the surface of the web by different methods and the excess of the treating agent layer is doctored off and the treating agent is evened out by a blade which is in contact with the web. In blade coating, as the application method it is possible to use, for example, applicator roll application or nozzle application. In film transfer coating, a desired treating agent layer is applied to the surface of a roll, from which the treating agent is transferred to the surface of the web in a nip between said roll and a backing roll. The amount of the treating agent layer is regulated by means of a doctor blade or a metering bar. In air knife coating, application is carried out, for example, by means of a roll applicator device or nozzles. The actual regulation of the amount of treating agent is performed by an air doctor. In connection with the air doctor there is additionally a mist chamber, the function of which is to collect the treating agent mist separated by the doctor and to separate treating agent particles from air.
By non-contact methods are meant methods in which the applicator device is not in contact with the surface to be coated and in which the amount of the treating agent applied to the web is not regulated any more separately after the treating agent has been applied. The non-contact coating methods include spray coating and curtain coating. In spray coating, the treating agent is pumped under high pressure through a number of small nozzles onto the surface of the web that is being treated. Small treating agent droplets impinge upon the web and spread through their own momentum on the surface of the web. In curtain coating, an even treating agent layer is formed which falls as a curtain under the force of gravity onto the surface of the web. Curtain coating devices can be divided into coating devices which feed from a gap or which feed from a planar surface. In applicator beams that feed from a gap, a treating agent is pumped through a distribution chamber into a narrow gap which is situated above the web and at the lip of which a curtain is formed and flows onto the web below. In the curtain coating devices feeding from a planar surface, a treating agent is caused to flow along an inclined planar surface to the edge of the planar surface, from which the treating agent flows in the form of a curtain onto the surface of the web below.
U.S. Pat. No. 6,063,450 discloses a method and a device for feeding a treating agent directly or indirectly onto one or both sides of a continuous surface. The device comprises a nozzle array, the treating agent being fed from its nozzles under pressure towards a deflection means, i.e. a curved impact plate. The direction of treating agent jets is changed by means of the curved impact plate such that the treating agent jets are directed at the surface of a web running on the outer shell of a supporting roll (direct application) or at the surface of the outer shell of the supporting roll (indirect application). In indirect application, the treating agent is transferred from the outer surface of the supporting roll to the web in a nip between the supporting roll and a backing roll. After the point where the treating agent impinges upon the surface of the web running on the outer shell of the supporting roll or upon the surface of the outer shell of the supporting roll, evening out of the treating agent layer and adjustment of its thickness are further carried out by means of a doctor roll placed against the supporting roll. In the embodiments described in the patent, the nozzle array and the impact plate are situated below the web and the supporting roll. The treating agent discharges from the nozzles in a horizontal direction or obliquely upwards with respect to the horizontal direction. From the trailing edge of the impact plate the treating agent discharges almost vertically or at a small angle to the vertical direction towards the web or the outer shell of the supporting roll. The kinetic energy of the treating agent discharging from the nozzles must be relatively high in order that this kind of application against the force of gravity should be possible.
U.S. Pat. No. 4,093,016 discloses a method for manufacturing a paper web formed of several layers. Pulp is fed onto a wire supporting the web at locations spaced from one another, whereby a web composed of superimposed layers is formed. In addition, starch is fed onto the upper surface of at least one layer from a separate device that applies curtain coating. The device comprises a chamber which is placed above the web and comprises a vertical side wall. This vertical side wall comprises a horizontally extending slot provided with a lower and an upper edge, the width of the slot being 1.6-12.7 mm in the vertical direction. The surface of the starch supplied into the chamber is kept in the chamber on such a level that it extends above the upper edge of said slot at all times. Starch is pressed out of the slot as a horizontal flow which is in contact with both the upper edge of the slot and the lower edge of the slot. The thickness of the flow is determined by the vertical width of the slot. The starch discharging from the slot falls, because of gravity, down the vertical surface beginning from the lower edge of the slot, which surface terminates at a trailing edge situated at a distance below said slot. The starch falling along the vertical surface forms a continuous curtain, which becomes attenuated when it falls downwards. The thickness of the curtain leaving the trailing edge of the vertical surface is thus smaller than the thickness of the curtain discharging from the slot. From the trailing edge of the vertical surface the attenuated continuous curtain drops onto the web below. The chamber may also be slightly turned such that said vertical flow surface forms an angle of about 10° to the vertical plane. The inclined flow surface makes it possible to reduce air entrainment in the film and stresses acting on the film.
EP Patent 609 535 discloses a curtain coating device that feeds along an inclined surface and uses a start plate. Before the coating operation is started, the start plate provided with an articulation joint is turned to a start position above the web. In the start position, a coating curtain impinges upon the inclined start plate and flows along it into a collection container. After that, the start plate is turned into a coating position, in which connection the coating curtain falls directly onto the web. The invention in this publication relates to the shape of the upper edge of the start plate, which forms one or more angles to the transverse direction of the curtain. In one embodiment, the upper part of the start plate comprises an L-shaped portion that receives the coating curtain and has a length of 2-10 mm, advantageously 3-5 mm. When the start plate is turned from the start position into the coating position, the coating curtain falls, in the intermediate position of the start plate, onto said L-shaped inclined portion of the start plate receiving the coating curtain and flows along it further onto the web. By these arrangements, attempts are made to ensure that the thickness of the coating layer that is being formed on the web does not increase during the initial phase of the coating operation. The publication mentions that one problem is that the thickness of the coating layer becomes larger during the initial stage of coating. The apparatus described in the publication is designed mainly for coating of a photographic film.
Metso Paper, Inc.'s FI Patent Application 991498, which is incorporated by reference herein, discloses an arrangement for spreading a treating agent on a moving surface in a paper or board machine. The arrangement comprises at least one feed chamber, into which the treating agent is fed, and members for guiding the treating agent from the feed chamber onto a moving web. Said means for guiding the treating agent comprise a nozzle plate, which closes at least partly said at least one feed chamber. The nozzle plate has holes which are defined by the nozzle plate around their entire periphery. By means of these holes, treatment jets are formed which are applied to the moving surface. The nozzle plate is formed of a thin plate which has at least one row of holes formed in it. The row of holes comprises holes that are relatively small and close to one another.
The feed apparatus disclosed in FI Patent Application 991498 is very simple and reliable in operation. The apparatus can be made very narrow, which means that it fits even in a gap between a roll and a coming web. Thus, the apparatus can be used at many locations where it has been difficult or impossible to apply a treating agent. In the feed apparatus, the feeding of the treating agent takes place through the holes of the thin nozzle plate. The holes are defined only by the thin nozzle plate, so the length of the holes in relation to the diameter of the holes can be made small. By this kind of arrangement the treating agent jets can be made uniform without formation of drops or mist, and the mass and the impulse force of the treating agent jet are sufficiently high for the treating agent to be transferred and attached to the surface that is being coated. In a situation in which the holes of the nozzle plate are relatively far from one another, the treating agent, when it discharges from the holes, forms separate single jets, which form a discontinuous treating agent curtain with a desired spacing in the cross direction of the moving surface. When impinging upon the moving surface, the single jets form an even uniform treating agent layer. In a situation in which the holes of the nozzle plate are very close to one another, the treating agent, when it discharges from the holes, forms a continuous treating agent curtain in the cross direction of the moving surface.
The arrangement described in this FI Patent Application 991498 operates in a very wide running speed range, and thus it is suitable as such for many kinds of sites of use. However, in certain conditions, there may be problems in connection with the arrangement.
In a situation in which the holes of the nozzle plate are relatively far from one another and attempts are made to produce treating agent jets which are separate from one another, the jet emerging from some hole may be formed incompletely or be directed askew, for example, because of the faulty edge of said hole. As a result of this, one or more individual jets may become constricted and stopped, with the result that streaks may be produced in the moving surface, i.e. the treating agent does not form an even layer on the moving surface. The problem manifests itself particularly at low flow rates of the treating agent.
In a situation in which the holes of the nozzle plate are very close to one another and attempts are made to produce a continuous treating agent curtain immediately when the treating agent discharges from the holes of the nozzle plate, the jet emerging from some hole may be formed incompletely or be directed askew, for example, because of the faulty edge of said hole. As a result of this, the continuous treating agent curtain may be broken, with the result that streaks may be produced in the moving surface, i.e. the treating agent does not form an even layer on the moving surface.